1. Field of the Invention
This invention relates to indicators for evidencing when frozen foods in freezer compartments have been subject to temperatures above a previously selected temperature for a sufficient time to result in deterioration of the food and relates, in particular, to indicators for use in frost-free-type refrigerators and freezers having heating devices which periodically heat the walls of the freezer compartment for a length of time sufficient to remove ice and frost from the walls.
2. Prior Art
The preservation of food during storage has long been accomplished by means of refrigeration which keeps the food in a frozen condition so that degradation by enzymes and bacteria is prevented. Those experienced in the science of refrigeration have found that frozen food must be maintained near 0.degree. F. and should not rise above 5.degree. F. for any extended period of storage or enzymatic or bacterial action or both can take place with the result that changes in taste and the development of toxic substances can occur. In many cases, power failure or improper operation of equipment have resulted in the temperature rising above 5.degree. F. for extended periods or even complete thawing has taken place. Subsequent resumption of power re-freezes the food and in many cases the thawing cycle escapes detection. It is, therefore, not known that the food may be in a dangerous condition for human consumption.
A number of devices have been invented to detect whether a thawing and subsequent freezing cycle have taken place. Most of these devices have made use of irreversible damage caused to emulsions, gels, frangible partitions, capsules, or of the deformation of cast geometrical shapes of ice. These types of inventions, being irreversible, can only be used once. Consequently, every time the refrigerator is defrosted, they must be replaced. They will also suffer permanent damage if subjected to freezing during storage or shipment. A lesser number of devices have been invented which are re-usable and in some cases would operate as described in conventional refrigerators. However, most modern refrigerators are of the frost-free-type which operate on the principle that, twice a day, they pass through a heating cycle to remove accumulated surface frost. During the course of this defrosting cycle the temperature in the refrigeration compartment rises between 40.degree. and 45.degree. F. and the elapsed time for the equipment to change from 0.degree. up to over 40.degree. F. and back down again to 0.degree. F. takes about 11/2 hours.
None of the pre-described devices which claim to be usable in temperature ranges of less than 0.degree. up to 32.degree. F., are usable in frost-free refrigerators unless they are replaced or re-set twice a day, as they are activated during each frost-free heat cycle. Consequently, it would be impossible, under normal usage of the refrigerator, to determine whether such inventions had been set off by improper operation of the equipment or whether they had been caused to function during the heating cycle. It is, therefore, the object of the present invention to provide a means of determining whether frost-free refrigeration equipment will maintain a pre-selected temperature between 0.degree. and 32.degree. F. during the periods between the frost-free heating cycles. The present device is not activated during the heating cycle and is re-usable so that it may be re-used in case of shut-down for repairs or if changing refrigerators. The present invention is based on both a unique engineering concept and a very unique harmless chemical solution which provides for successful operation under exposure to the heat cycle in frost-free equipment.
Super-cooling of liquids in freeze/thaw detectors is a major problem as the well known methods of starting crystallization, such as stirring, shaking, seeding, etc., are not practical in such devices and even the addition of substances, such as silver iodide can be ineffective. Glycols, alcohols (eg. benzyl alcohol), o-dichlorobenzene for example, can be kept for weeks at temperatures lower than their published freezing points without solidifying. Tests have indicated that aqueous solutions of glycols, sucrose, sorbitol, mannitol, alcohol, glycerol, sodium chloride, calcium chloride, magnesium chloride, acetic acid, and ammonium chloride formed slushes when frozen and were difficult, even with extensive freezing, to form a homogeneous solid which did not have a portion of liquid liquor within its mass. A solution of ammonia in water was found to freeze sharply and solidly, but it was found that it, along with all the above chemicals and solutions, melted readily when subjected to even one heat cycle of frost-free refrigerator. It was also found, that when such solutions were adjusted to freeze between 0.degree. and 26.degree. F., that the solutions that had the lower freezing points were much quicker to melt during the frost-free cycle. Samples of vegetable oils and fats were selected which froze sharply between temperatures of 0.degree. and 14.degree. F. but were found to melt more readily in the frost-free heat cycle than many of the chemicals tested. Aqueous glycol solutions and vegetable oils were placed in separate vials so that the vials were half-filled. These vials were half imbedded in 21/2inch cubes of foamed urethane insulation and frozen. When these insulated samples were exposed to the heat cycle of a frost-free refrigerator, all samples were readily melted.